What could cause a car to stutter at low speed.?

I would say the throttle position sensor (TPS) could have a dead spot in it. There should be a three wire connector plugged into it. This sensor receives a 5 volt reference signal from the ECU, and the TPS alters that signal based on the position of the butterfly valve in the throttle body in which it has a direct mechanical connection to the valve hinge.

Using a multimeter, you need to determine which wire is your 5 volt reference, which is your signal return, and which is your ground, and you will need to back probe the signal return and ground wires.

To find the 5V reference wire, set your multimeter to 20V DC, unplug the TPS connector and turn the key to the ON position. Ground one lead of your multimeter to battery negative, and use the other lead to probe the three leads on the TPS connector. When the multimeter shows 5 volts, you have found the 5V reference wire. Leave that wire alone at that point.

To find the ground wire, change your multimeter setting to Ohms and check the remaining two leads on the connector. Be careful not to touch the 5V reference lead as that will cause a short to your multimeter, potentially ruining it. The signal return wire should be self explanatory at that point.

Next, plug the connector back into the TPS and using something like T-pins or safety pins, back probe the ground and signal return wires, with the leads from your multimeter connected to the pins and it set back to 20V DC.

The voltage range of the TPS is typically 0.5V at rest and 4.5V wide open. Gradually move the throttle valve open and watch the multimeter. The voltage should steadily increase with the movement. If at any point the voltage suddenly drops to 0, that means there is a dead spot in the TPS and it will need to be replaced.

Hope this helps.

Check for trouble codes.

I'd start with AC iridium spark plugs and plug wires. Make sure the spark plug tower contacts are not rusty. Remove any rust with sandpaper.

Clean the throttle body too.

I've owned 4 GM's with the 3,8 liter supercharged motor.

Throttle Position Sensor.

Head gasket